This invention relates to the field of wide format color digital printing systems, and in particular to an improved drum-based ink jet printer having a novel service station for precisely cleaning and storing a plurality of marking elements associated with ink emitting print heads. The present invention finds use in the digital printing and imaging industry where successful repeatable printing requires precise placement of dots on print substrates including paper, vinyl, film, and other print substrates of varying thickness that typically include emulsion coatings thereon to promote consistent interaction with printing media marking materials such as ink, toner, or various printing compounds.
The present invention relates to an improved apparatus for supporting, driving, maintaining, and operating print heads coupled to a carriage across a printing media so that ink droplets may be placed thereon to reproduce text and images with precision. Prior art mechanisms for driving a print carriage typically utilize a belt and pulley actuated mechanism or helical gear attached to the print carriage. These prior art carriage mounting and drive systems are known to cause errors in printed output.
Prior art print head rail members for large format print engines further encounter limitations due simply to the length and mass of a typical rail, carriage drive means, and carriage assembly as well as mechanical control difficulties related to precisely controlling all the specifications and tolerances during manufacturing and installation. A phenomena described as xe2x80x9ctolerance stackingxe2x80x9d contributes a significant component of error in an assembly process wherein at least two precision machining events occur at differing times on the same assembly or same time for different assemblies. In relation to a carriage assembly for a large format print engine, such tolerance stacking occurs at a number of discrete points of manufacture. For example, a carriage typically precisely supports at least one, and oftentimes several, print heads, a portion of the circuitry for such print heads, and all intervening means for driving the carriage assembly upon a guideway or track in a highly controlled manner (e.g., belts, pulleys, gears, encoders, motors, etc.). Thus, machining portions of subassemblies to arrive at an exact location of the print head(s) relative to: other print head(s), the printing medium, and the rail assembly can all contribute positional error relative to a design criteria possessing rigorous tolerance specifications. Prior art carriages typically couple to opposing sides of a single rail and often include a driving force to both impart axial motion and to constrain movement of the carriage. The inventors believe that using an attachment point for driving a carriage causes errors in the locations of ink droplets on the paper as described hereinbelow.
Additional problems with prior ink jet head configurations involve the mounting of the print head for accurate placement and movement across the printed image. The rail structure for the print head must adequately support the print head not only over the entire printed image, but also for any cleaning, maintenance, and temporary storage of the print heads. It is common to provide a zone, away from the printing medium within which to xe2x80x9cparkxe2x80x9d the print head to perform auxiliary xe2x80x9cservicexe2x80x9d functions, this zone is commonly known as a service station. These auxiliary functions may include manipulating the carriage, certain calibration functions, cleaning and capping of the print heads. To accommodate the park zone, the support system, or rails, must support the head over a distance greater than the width of the printing medium. For example, printers handing printing medium about 36 inches wide may have rails about 48 inches long.
Accurate placement and movement of the print head becomes more and more difficult as the length of the print scan (i.e., the width of the image) increases. Most prior ink jet printers over about 17 inches wide employ either a two-rail structure, or a single-rail and outrigger structure, for head carriage X-directional travel. Both of these techniques provide two separate and independently adjustable support points for the carriage. Multiple support systems were used on wide format printers because it was believed that a single rail could not provide adequate support and stability for the print head over a large distance. Multiple support systems were utilized to provide a wider support base for the print head and carriage to lessen the effect of any stability problems, as well as to provide additional strength to lessen rail flexing problems. Vibration problems may occur if the print head undergoes movement with respect to the rail structure. The print head may slightly rotate or shake about an axis parallel to the rails, causing the print head placement with regard to the paper surface to be inaccurate. Alternatively, the print head may slightly rotate or shake from side to side on the rails, perhaps due to the direction of print head travel.
Dual support systems have not proven to be altogether feasible for graphics quality large format printing because it is generally difficult to maintain parallelism of the supports across the entire width of the large format media and oftentimes either more or less than three discrete points constrain the carriage. More particularly, each support introduces positional error, resulting in non-parallel guide paths for the carriage. Further, prior art two-rail systems employ a pair of circular rails, with the print head mounted on a carriage which is in turn mounted on the rails. The carriage is generally supported by circular sets of ball bearings wrapped around each of the circular rails. Non-parallelism of the rails introduces vibration through the ball bearings to the carriage, often causing instantaneous horizontal velocity errors. If the supports are not parallel, the rollers on the carriage will bind or have excess freedom at particular locations along the rails, and cause further stability and vibration problems. If bending of the rails occurs and the railings are not maintained completely straight, errors occur in positioning the print head. Additional problems occur due to the space that the rails take up, interfering with the transfer of electronics and ink from the printer housing to the print head. It will be appreciated that these problems are magnified as the length of the rail or rails becomes greater, as in large-format printing. Accordingly, a print head configuration is desired which will avoid these various problems.
Furthermore, such prior art carriage and drive systems typically are not designed for in-field replacement with minimum personnel and requiring a minimum amount of service time. In fact, due to the obvious competing design objectives of mechanical positional accuracy and field replacement convenience, the inventors are aware of only one other such rail system offering similar design benefits as the present invention. The inventors refer to U.S. Pat. No. 5,592,202, issued Jan. 7, 1997 and titled xe2x80x9cInk Jet Printer Rail Assemblyxe2x80x9d which application is commonly assigned with the present invention to ColorSpan Corporation (formerly known as LaserMaster Corporation) of Eden Prairie, Minn. 55344. In the application cited, a single rail pivotably attaches to at least one end of a print engine chassis so that the carriage riding thereon may be removed for field service and replacement. The benefits of such a system relate to diminished down-time, reduced required service, and efficient repairs, thereby reducing the overall cost of ownership involved in operating one or more large format digital print engines.
Prior art digital printing systems typically operate by moving a print medium transverse to an articulated imaging print head. The print head frequently includes a plurality of discrete imaging elements suitably arranged in a pattern wherein one or more linearly aligned sets of ink emitting nozzles, or jets, disposed perpendicular to the direction of movement of the printing substrate for providing maximum coverage. The net result is that discrete dots are placed on precise locations on the printing media so that a pleasing visual image is rendered upon the printing media. However, as the number of print heads (and therefore jets) increases registration among the different print heads become more complex at the same that it becomes more crucial to the quality of the printed output formed by the printer. A picture element or pixel generally refers to a coverage area on the print media (or a sensing element in an imaging array) defined by resolution in a xe2x80x9cverticalxe2x80x9d direction relative to a xe2x80x9chorizontalxe2x80x9d direction. These pixels must be controlled very carefully to impart desired quality of the image, and the physical and chemical interaction between the marking material and the printing media and the environmental conditions under which the marking material is deposited upon the printing media all contribute to the quality of the actual image rendered. In fact, due largely to media characteristics, the inventors prefer to utilize a single pass printing mode wherein all the image pixels are formed in a single pass of the carriage assembly over the printing substrate to avoid such problems.
Most digital print engines that typically use one or more of a subset of the four subtractive primary colors: cyan, magenta, yellow, and black (xe2x80x9cCMYKxe2x80x9d) and rely upon color blending of these four ink colors to achieve accurate representations of desired color(s). Upon combining ink colors at a given pixel that a particular color combination can be formed by having multiple ink colors at a particular pixel location, either in a dot-on-dot or a dot-next-to-dot configuration. In sum, digital printing processes involve precisely placing a number of tiny dots onto particular locations on a printing medium. Any number of these small dots, when viewed some distance away from a printing medium such as film or paper, are perceived as a continuous-tone visual image and means for registering the dots is crucial, although only x-y registration was heretofore available. Thus, it can be appreciated that even slight variance in the actual positional location of the ink dots can significantly effect the overall visual impression created by the printed image. In one subset of digital printing technology, aqueous ink is expelled from a plurality of ink jet nozzles to form dots on the printing media via thermal, piezoelectric, acoustic, and other technologies used for reliably ejecting a tiny droplet of ink onto a printing substrate. This is known as xe2x80x9cink jetxe2x80x9d printing and its popularity and the innovation related thereto have greatly increased the accuracy and therefore the photorealistic quality of the images printed, while at the same time attempting to lower the costs of ownership as larger and larger wide format print engines are commercially introduced. While the types and numbers of inks, and ink jet cartridges, usable with such printers have increased thereby increasing the complexity of controlling interaction among the inks, cartridges, and printing medium, reduced costs of ownership and ease of serviceability continue to drive a large amount of innovation in this field of endeavor. Thus, a continued need exists in the art for low cost and at the same time technically advanced, highly accurate means of performing wide format color digital ink jet printing.
The reader is encouraged to cross reference and review the present document with a number of [now allowed] U.S. patent applications, each filed on Sep. 10, 1996, each commonly assigned, and the contents of each are hereby incorporated by reference in their entirety herein; specifically, U.S. patent application Ser. No. 08/711,796 entitled xe2x80x9cCooperating Mechanical Sub-Assemblies for a Drum-Based Wide Format Digital Color Print Engine;xe2x80x9d U.S. patent application Ser. No. 08/709,803 xe2x80x9cCalibration and Registration Compensation Method for Manufacturing a Drum-based Print Engine;xe2x80x9d U.S. patent application Ser. No. 08/709,804 xe2x80x9cMethod for Manufacturability of a Low Cost Printing Drum;xe2x80x9d U.S. patent application Ser. No. 08/709,803 xe2x80x9cMethod and Apparatus for Compensating for Faulty Ink Emitting Elements in a Digital Output Device;xe2x80x9d and U.S. patent application Ser. No. 08/711,815 xe2x80x9cMethod of Selecting an Ink Set of an Ink Jet Printer.xe2x80x9d Furthermore, applicant herein incorporates by reference U.S. patent application Ser. No. 08/922,297 xe2x80x9cMethod and Apparatus for Registration and Color Fidelity Control in a Multihead Digital Color Print Enginexe2x80x9d which was filed on Sep. 3, 1997; U.S. Pat. No. 5,369,429 titled xe2x80x9cContinuous Ink Refill System for Disposable Ink Jet Cartridges Having a Predetermined Ink Capacity;xe2x80x9d and U.S. Pat. No. 5,469,201 directed to a guideway for a continuous ink refill system, all three of which are also commonly owned by the present assignee.
The present invention relates to a low cost, large format print engine featuring field replaceable subassemblies such as: a carriage having a vertically-articulated articulated portion (which retains a plurality of print heads and an imaging sensor array) and carriage drive means preferably comprising a combination rail and mechanically isolated lead screw assembly, a motor to both drive a printing drum and provide reliable means for monitoring and controlling drum rotation, two electronics subassembliesxe2x80x94a first for operating the printing system software (disposed in an electronics bay), and a second for handling all print related image data management and printing operations (disposed on the carriage assembly), and a service station subassembly for cleaning and maintaining a plurality of ink jet cartridges operating in said large format ink jet print engine. The print engine described with reference to preferred embodiments herein solves prior art difficulties and offers a new series of low-cost accurate wide format print engines. The print engine of the present invention contains a CD-ROM reader, hard drive, and Intel Pentium Processor chip and related circuitry so that relatively complex large format digital color computerized printers as described and enabled herein may be reliably and simply fabricated, operated, and serviced and thereby producible in high volume at reduced cost making ownership of such machines less expensive overall. Some of the key advantage of a print engine taught herein include; a two stage carriage assembly having a vertically articulated portion housing ink jet print heads and preferably an optical imaging array; an extremely resilient three-point (planar) stance where the carriage assembly rides on the printer rail structure; a mechanically-isolated attachment to a lead screw drive means; and an improved service station assembly.
The improved drum-based printer of the present invention again utilizes virtually no critical parallel alignments; in that the inventors have again implemented a pair of complementary alignment plates that are virtually impervious to errors during manufacture and which posses extremely robust behavior in almost every orientation. In fact the two alignment plates of the present invention must practically be physically bent, or grossly out of relatively parallel alignment before any of the sub-assemblies of the present invention are affected. Thus, the alignment plates handle a great deal of chassis movement without deleterious effects on the head height relative to the print medium.
The present invention may be performed with most types of large format digital print head technologies since the characteristics of the print head are more or less independent of the manner in which a carriage for conveying a print head or heads across a print media to create a large format digital image. Thus, the present invention encompasses a carriage assembly which supports the print heads in close proximity to a print medium via a vertically articulated portion of the carriage assembly and couples to a helical lead screw member at a mechanically isolated engagement point to reliably control the transverse motion of the carriage relative to the printing medium.
In a preferred embodiment, the carriage contacts three support points, thereby forming an extremely stable planar foundation from which deflection is unlikely and is not supported but is rather only driven via engagement at a single drive location by a novel, spring-biased helical lead screw for reliably propelling the carriage with a minimum of resistance. In this embodiment, the drive location is preferably spring-biased to one side of the grooves that form the driving portion of said lead screw and provided with a single-degree of freedom spring-biased attachment sub-assembly. This attachment subassembly imparts an axial force that absorbs a surprisingly large amount of non-axial vibration and movement, which are potential sources of tracking error for the entire carriage assembly particularly when the lead screw reverses rotation. Furthermore, the three-point stance and minimal rolling resistance absorbs an unprecedented amount of force without deflection. Thus, heavy pre-printing, printing, or post-printing assemblies and associated electronics can be easily accommodated with this design. Also, the service station assembly has an independent suspension for all xe2x80x9cstationsxe2x80x9d and uses the print heads themselves for guidance accuracy during service station operation. Finally, the vertically articulated print heads can adjust to accommodate myriad printing substrates of vastly differing thickness.
The following drawings are representative only and as such should be viewed as illustrative and not limiting to any particular embodiment of the present invention or of any relative scale of the features depicted therein.